The aim of this project is to develop strategies to manufacture functionalized sorption materials and test them for application in a new type of 225Ra/225Ac and 225Ac/213Bi generator. The target material has the following characteristics: (1) high separation factors (SFs) for the ion pairs (αRa/Ac and αAc/Bi), (2) high resistance against radiolysis, (3) high sorption capacity and (4) uniform spherical shape with narrow size distribution to allow optimal column packing.
The work flow is composed of synthesis of the functionalised material followed by a systematic evaluation of the abovementioned criteria, starting with the most essential one. This way, each new material will go through the following cycle of five stages: (1) synthesis (2) evaluation of separation chemistry (3) evaluation of radiation resistance (4) final shaping into uniform spheres and (5) test and validation of the generator. This flow chart is schematically presented in Figure 1.
Figure 1 - Stages involved in production and evaluation of new inorganic sorption materials for radionuclidic generators for medical applications
Synthesis and surface modification of sorption materials
The main selection criteria for the sorption material are dominated by the harsh experimental conditions, imposing a.o. a high radiation resistance and acid stability. By combining a tuned pore structure with specific surface modifications, a superior separation performance is targeted.
A first class of selected materials are activated carbons, which are widely recognised as sorbent materials due to the variety in pore structure and surface chemistry . Its non-crystalline structure is indicative for a high radiation resistance. In literature, several approaches have been described to form a (meso)porous carbon (composite) material and to activate its surface, either by thermal, chemical or plasma treatment . Depending on the source materials and on the conditions of the activation, a range of functional groups can be formed on the surface (either containing oxygen, nitrogen, phosphor or sulphur containing species)  . These surface groups can then be used as anchoring points for further chemical functionalisation. As such, the designed surface chemistry leads to improved separation performance, pH stability (low pKa) and radiation stability (aromatic functionality). For a first generation material, two activated carbon materials will be prepared with different properties and functionalized with two chemical groups (resulting thus in four different first generation materials).
A backup class of materials could be functionalized mesoporous TiO2, which have been applied in the selective sorption of actinides . These materials demonstrate a superior hydrolytic resistance and radiation stability compared to a.o. polymer-based resins .
Evaluation of separation chemistry with respect to Ra/Ac and Ac/Bi separation
Batch experiments will be performed to determine distribution coefficients (KD) for cations of interest between solid and aqueous phase as a function of the aqueous phase composition (pH, mineral acid type and concentration, salinity, concentration of other ions) and temperature will be performed. Initially, “cold” tests will be performed with Ba as simulant for Ra and La as simulant for Ac, followed by “hot” tests using Ra and Ac tracers. The final result will be the determination of SFs in function of the abovementioned parameters.
- Evaluation of radiation resistance
Suspensions of sorption materials that successfully passed the second stage, will be exposed to gamma radiation up to a dose of ca. 20 kJ/kg of sorption material in either the spent fuel or 60Co gamma irradiation facility at SCK•CEN. Since radiolytic damage to ion exchange resins is higher when exposed to gamma radiation compared to alpha radiation, no radiolysis studies using alpha irradiation will be performed . Afterwards, batch experiments to evaluate SFs, SEM for particle morphology analysis, FT-IR spectroscopy for functional group analysis will allow to evaluate the radiation resistance of functionalized sorption materials. Only those materials that are sufficiently resistant will enter stage four of the cycle.
- Shaping of successful material into porous powder with uniform characteristics
Based on the separation performance and data of the radiation resistance, 1 or 2 materials will be selected to be shaped by controlled coagulation into spherical microspheres. The structural uniformity and spherical shape of the sorption material will be reflected in an improved column packing and elution characteristics . In order to balance the mechanical properties and the porous architecture, different shaping approaches can be developed, either based on the use of inorganic binders or the carbonisation of polymeric templates. The porous architecture of the microspheres will be analysed by a combination of analytical tools including N2 sorption, Hg porosimetry and SEM.
Once shaped, the knowledge on the surface functionalisation of powders will be transferred to that of the microsphere.
Test and validation of generator
After shaping, the selected materials are evaluated by column chromatography for the cold system under conditions based on the KD values determined in stage 2 for Ba, La and Bi. Finally, column hot chromatographic experiments with Ra, Ac and Bi will be performed to evaluate and optimize Ra/Ac and Ac/Bi separations, taking into account the presence of 209Pb, which is a daughter nuclide of the series.
Stages 1 and 4 will be performed at VITO, stages 2, 3, and 5 will be performed at SCK•CEN. All work will be done with strong interaction of the university promotor of KU Leuven.
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